This is really confusing me...
EPA Requirement: The only law (passed by the U.S. Congress) regulating how to use the NRR, is actually the EPA labeling regulation, dating back to 1981. This law directs the user to deduct the NRR from the noise level at hand, in order to get the noise level at the ear. That is pretty straightforward: Noise level [98 dB] minus Hearing Protector NRR [25 dB] = Noise at the Ear, 73 dB.
OSHA's Field Manual: Although OSHA has no legislative authority, the agency has in its field manual directed their inspectors to use the following formula for how to apply the NRR.
Conflicting Information about How to Use the NRR
- Example, the tool and hearing protector discussed above: 100 dB - 25dB = 75 dB at the ear.
- For professional applications, when the noise level is known in dB(A), OSHA advises that the NRR be reduced by 7 dB:
- Example: 100dB(A) - (25 - 7)dB = 82dB at the ear.
- OSHA may also require a 50% reduction of the NRR after the above reduction:
- Example: 100dB - (25 - 7) x 0.5 = 91dB at the ear.
EPA Requirement: The only law (passed by the U.S. Congress) regulating how to use the NRR, is actually the EPA labeling regulation, dating back to 1981. This law directs the user to deduct the NRR from the noise level at hand, in order to get the noise level at the ear. That is pretty straightforward: Noise level [98 dB] minus Hearing Protector NRR [25 dB] = Noise at the Ear, 73 dB.
OSHA's Field Manual: Although OSHA has no legislative authority, the agency has in its field manual directed their inspectors to use the following formula for how to apply the NRR.
- If noise measurements are made with the dB(A) scale, the following formula applies. (Noise level [98 dB] minus NRR divided by 2 [25 dB minus 7 dB divided by 2] = Noise level at ear, 89dB).
- If the noise level measurements are made with the dB(C) scale, this formula applies. (Noise level [98 dB] minus NRR divided by two [25 dB divided by 2] = Noise level at ear, 85.5 dB).
- Although OSHA has only enforcement authority, their interpretation has held up in court. OSHA only recommends that the 50% safety factor is applied. Realize that if a company runs an effective hearing conservation program, they are not likely to be challenged. An effective hearing conservation program can be defined as a program that assures that no new permanent and noise induced hearing losses occur among its employees.
- Earmuffs, Subtract 25% from the manufacturer's labeled NRR
- Formable earplugs, Subtract 50% from the manufacturer's labeled NRR
- All other earplugs, Subtract 70% from the manufacturer's labeled NRR
Conflicting Information about How to Use the NRR